1. Field of the Invention
This invention relates to novel volatile liquid reagents which can replace less satisfactory solid sources in film deposition processes such as chemical vapor deposition (CVD), spray coating, spin coating or sol-gel deposition. These liquid reagents can be used for deposition of materials containing alkali metals, such as metal oxides or mixed metal oxides.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a widely-used process for forming solid materials, such as coatings or powders, from reactants in the vapor phase. Comprehensive reviews of CVD processes have been given recently in “CVD of Nonmetals,” W. S. Rees, Jr., Editor, VCH Publishers, Weinheim, Germany, 1996; “CVD of Compound Semiconductors,” A. C. Jones and P. O'Brien, VCH, 1996; and “The Chemistry of Metal CVD,” T. Kodas and M. Hampden-Smith, Editors, VCH, 1994.
In CVD processes, a reactant vapor may be created by heating a liquid to a sufficiently high temperature and bubbling a flow of a carrier gas through the liquid, to transport the vapor into the CVD chamber. In a low-pressure CVD system, the carrier gas may be omitted, and the vapor may flow directly from the bubbler into the low-pressure CVD chamber.
In order for a CVD process to function successfully, it is necessary to create a vapor containing controlled amounts of suitably reactive chemicals. Solids can be used as sources of vapor in CVD processes. However, when solids are used in a bubbler, the rate of vapor production by sublimation of a solid is not easily reproducible, because the amount of vapor produced often depends on the particle size and shape, which change as the sublimation process continues. Thus the vapor concentration can change in an uncontrolled way, thereby changing the growth rate and/or the composition of materials made by the CVD process. Also, different batches of solid may have different sizes and shapes of particles, so that the results of a CVD process may change when a new batch of solid precursor is placed in the system. These difficulties are particularly evident in the currently-used solid CVD precursors, lithium 2,2,6,6-tetramethylheptane-3,5-dionate, often abbreviated Li(thd) or Li(dpm) and potassium 2,2,6,6-tetramethylheptane-3,5-dionate, K(thd), used by C. Kiyofumi, A. Onoe and A. Yoshida, Jpn. J. Appl. Phys., Part 1, vol. 37, pp. 5582–5587 (1998) and R. S. Feigelson, J. Cryst. Growth, vol. 166, pp. 1–16 (1996). Solid lithium tert-butoxide, LiOtBu, was used in the CVD of lithium niobate by A. Tanaka, K. Miyashita, T. Tashiro, M. Masakazu and T. Sukegawa, J. Cryst. Growth, vol. 148, pp. 324–326 (1995). Solid sodium hexfluoroisopropoxide was sublimed to provide vapors for CVD of sodium fluoride by L. J. Lingg, A. D. Berry, A. P. Purdy and K. J. Ewing, Thin Solid Films, vol. 209, pp. 9–16 (1992). None of these prior art sources for CVD of alkali metals are liquids at room temperature.
Another problem with solids is that their rate of sublimation can be altered by small amounts of contamination on their surfaces. In contrast, liquid surfaces tend to be refreshed by motion of the liquid, so that they tend to evaporate at a reproducible rate even in the presence of small amounts of contaminants.
Some solid materials show different vapor pressures, depending on the history of how the particular sample was prepared or how long it has been stored. For example, barium 2,2,6,6-tetramethylheptane-3,5-dionate, Ba(thd)2, has been used to deposit barium strontium titanate (BST) films. Solid Ba(thd)2 exists in a number of oligomeric forms, ranging from trimers to tetramers to polymers of various lengths, depending on the method used for its synthesis. The rates of interconversion between oligomeric forms are slow, often taking weeks or months. Thus the molecular composition of a sample of Ba(thd)2 depends on how it was made and how long it has been stored. The vapor pressures of these oligomers are different from each other. Thus it is very difficult to predict the vapor pressure of any particular sample of Ba(thd)2 and the deposition rate of BST from this solid source is not reproducible. In comparison, liquids usually exist in only one reproducible form at any given temperature and pressure.
Another difficulty with solids is that rates of sublimation are often low, so that sufficiently high vapor concentrations cannot be produced. For example, K(thd) has a very low vapor pressure, which limits the deposition rate to low values. In comparison, liquids often have higher vapor pressures than solids. Another practical difficulty with solids is that transferring them between containers is less convenient than pumping liquids.
Thermal decomposition of solids is another problem that often affects the reproducibility of solid vapor sources. For example, solid K(thd) gradually decomposes at its sublimation temperature, so that the amount of vapor generated decreases with time. Thermal decomposition is also a potential problem for liquid sources, but its effect may be minimized for liquids by rapid or “flash” vaporization. This can be accomplished by pumping the liquid at a steady, controlled rate into a hot region in which the liquid vaporizes quickly. In such a “direct liquid injection” (DLI) system, each part of the liquid is heated for only a short time, and its vapor can be formed without significant decomposition even from thermally sensitive liquids. Another advantage of a DLI system is that multi-component mixtures can be vaporized in a fixed and reproducible ratio, even if the components differ in volatility. Because of these advantages, DLI systems are becoming more widely used in CVD processes.
Solid sources can be used in DLI vapor sources if a suitable liquid solvent can be found to dissolve the solid. However, solvents can introduce other difficulties, such as increased flammability, toxicity or corrosiveness of the precursor solution, increased incorporation of carbon or other impurities into the deposited film, and an increased volume of gaseous byproducts must be removed from the exhaust gases to avoid pollution.
Because of all these difficulties, solid sources of vapor are seldom used in commercial CVD processes. Sources that are liquid at room temperature are more convenient, and are almost always used in the practice of CVD where available. Creating a vapor from a liquid source would be much more reproducible and convenient than creating it from a solid source; however, there are no previously known volatile compounds of the alkali metals that are liquid at room temperature.